This invention relates to optical viewing systems. In particular, the invention relates to an optically guided long-range probing system which includes a coarse approach system and a magnifying probe.
Magnifying probes are known in the art. One type of such probe is a scanning tunneling microscope (STM), as described in Kuk and Silverman, "Scanning Tunneling Microscope Instrumentation," 60 Rev. Sci. Instrum. 165-180 (Feb. 1989), incorporated herein by reference. Such devices allow the surface structure of conductive materials to be observed at the atomic level. Topographical images and electrical characteristics of surface features may be obtained with lateral resolution less than 100 pm. In typical operation, the conductive probe tip of the STM is placed within approximately 1 nanometer of the conductive sample material. By applying a voltage between the conductive sample material and the conductive probe tip, the STM induces a current ("tunneling current") between the sample and the probe tip. The surface to be viewed must thus be conductive. This tunneling current is highly sensitive to extremely fine distance changes between the sample and the probe tip. By monitoring the tunneling current, the STM allows the surface structure of the sample to be observed at the atomic level.
Another type of magnifying probe is an atomic force microscope, or AFM, which is a combination of the principles of the STM and the stylus profilometer. Unlike the STM, the AFM does not require a conductive surface on the sample, and thus can be used to investigate both conductors and insulators on the atomic scale. Lateral resolutions of 30 .ANG. and vertical resolutions of less than 1 .ANG. are possible with the AFM. The operation and structure of ATM's are described more fully in G. Binnig and C. F. Quate, "Atomic Force Microscope," 56 Physical Review Letters pp 930-33 (1986), the disclosure of which is herein incorporated by reference.
One disadvantage of conventional magnifying probes is their inability to determine the location of the probe tip with respect to particular features on the sample surface. This is due in part to the limited range of view provided by a magnifying probe (typically 100 .mu.m.sup.2). Therefore, coarse approach systems have been designed, which enable the operator to position the probe tip of the magnifying probe at a desired location with respect to the sample. The magnifying probe itself may then be used for very fine positioning of the probe tip.
Previous attempts to create a coarse approach system for a magnifying probe have been too complex, expensive, or invasive. One known technique combines an STM with an electron microscope in a vacuum. The electron microscope serves to locate the coarse position of the probe tip with respect to the sample surface. This system, however, is undesirable because of its complexity and the cost associated with creating and maintaining the vacuum environment.
Piezoelectric-transducer scanners can be used for micrometer-scale scanning; however, for coarse positioning of the sample relative to the probe tip, other devices (which are capable of millimeter or greater scanning range) are used. The principal extant problem for measurements of a submicrometer area at a desired address in a macroscopic sample has been the initial positioning of the probe tip. That is, for characterization of relatively large nanostructures such as quantum dots, the issue is probe placement at the origin of a coordinate system to which a map of the heterostructure is referenced. Once this initial (coarse) placement is accomplished, the piezoelectric transducers can be used for fine positioning and scanning. For the characterization of relatively-large nanostructures, atomic resolution is usually not as important as optically-guided initial positioning; thus, probe-design considerations are different from those of the traditional magnifying probe.
Optical microscopes have been suggested as coarse positioning devices. However, such devices are undesirable because they add vibration to the system. Moreover, optical microscopes are limited in their degree of proximity to the tip-to-sample junction because of physical constraints.